You're asking for the roadmap, together with the timeline. From this I'm implying that you don't know what's on the roadmap that you absolutely need. And for this reason alone, my advice is to go with 6.2. Evaluate 7.0 once it's out. There will be a migration path and documentation on how to update your site .

If this isn't enough, here are some more reasons: 7.0 won't be out very soon. It sounds like you can wait quite a bit (as you even mention 7.1) with the start of your project. In that case you don't seem to need the resulting portal at all - so don't bother (Sorry if this sounds harsh - I'm trying to over-stress the point to make my opinion undoubtly clear).

Yes, there is a roadmap for 7.0, I'm not sure how reliable or public it is, but there are some milestones already out (you'll find them when you look for Jorge Ferrer's blogs) These blog articles might also give some hints on the remaining milestone builds before 7.0 will be out. Unfortunately I don't have anything to point to for the roadmap, but someone else might step in.

I'm not deeply involved with planning or engineering for the next version, thus I don't dare quoting any date that I've heard being discussed - it'd be wrong anyways. I'd argue that you should do the same. The only thing that you can be sure of is that 7.0 won't be out within the next few months. If it is: Party. But don't wait for something that you don't even know what it can do. This, by the way, applies to any unknown future software releases, not just to Liferay. The next version will always look brighter, better and more colorful. If the current version can do what you need: Go! Use the current version. Prepare for frequent upgrades - either minor or major. You should be able to do that anyway.

Again, I might have exaggerated a bit, this is purely to give you a clear answer rather than a typical "it depends".